I've got my US sewing machine running through a voltage converter at home, and have done so for about 7 years now (my serger and coverstitch are UK models though). What's important here is the watts your transformer can handle vs what your machines are using. When I first went through this, I had just a little one from RadioShack that melted down, too. I then went and bought the much bigger and heavier one I have now (I think it cost me about £60?) and it's worked great since day one.
So I'll have a look at mine tonight, but two important questions are:
1. Can you look in your manuals and figure out how many watts they each consume?
2. Will you be running both at the same time? (or will you do what I do and switch back and forth between them so you've really only ever got the power draw of one plus the other's lightbulb...)